1. Field of the Invention
This invention relates to an optical disk apparatus that detects a focus error signal by the astigmatism method.
2. Description of the Prior Art
In a known optical disk apparatus, laser light emitted from a laser light source is converged onto an optical disk serving as a recording medium. Reflected light from the optical disk is received by a photodetector to produce electric signals corresponding to recorded information and/or error information. In order to maintain accurate recording and/or reproducing the laser light must to be converged into a light spot the diameter of which is as small as a diffracting limit on the optical disk; accurate tracking of the beam is also required to keep the spot on the center of a track of the optical disk.
The optical disk apparatus has a detecting system to detect a focus error signal and a track error signal and a driving system to drive an objective lens along an optical axis direction and along a radial direction of the optical disk.
There are several methods to detect the focusing error signal. In general, an astigmatism method is used. The apparatus using the astigmatism method has a cylindrical lens placed in front of the photodetector. The light beam introduced on the photodetector will be circular only when the focus is correct, otherwise it becomes an ellipse where the aspect ratio of which changes as a function of the state of focus.
The photodetector is split into four photodetecting areas, and the focus error signal is generated from the difference between the outputs of the photodetecting areas.
However, when the focus error signal is detected by the above-mentioned astigmatism method, if the light beam has a wave front aberration, noise occurs in the focus error signal when the spot moves across a track.
The noise occurs as a result of a non-uniform intensity distribution, due to diffraction of the spot, when the light spot has a wave front aberration. In this specification, this noise is termed as F/T(focus/track) cross talk.
When the F/T cross talk occurs, the focus error signal indicates the objective lens being out of focus, even when the lens is in focus. Since the objective lens is driven along an optical axis direction, the objective lens is unstable whenever the light spot moves across a track. Therefore, the focus error signal is confused and movement of the light spot becomes inaccurate.